Field of the Invention
The invention relates to a method for determining the fill level of a medium in a container.
Description of Related Art
The medium is, for example, bulk goods or a liquid and the container is, for example, a tank, a silo or a channel.
It is known in industrial process automation either to continually measure the fill level of the medium or to specifically monitor particular limit levels.
Such devices for monitoring limit levels are also called fill level or limit level switches. They are used, for example, depending on the mounting position, for overflow protection, idle state protection or pump protection.
If the medium reaches a predetermined fill level or falls below such, the switches generally generate a signal that, e.g., interrupts a fill process or triggers a safety mechanism or causes the closing of a drain valve.
It is known, for example, to detect limit levels capacitively (e.g., German Patent Application DE 100 23 850 A1), using conductivity measurement (e.g., International Patent Application Publication WO 97/37198 A1 and corresponding U.S. Pat. No. 5,861,811) or using mechanically swingable sensors (e.g., German Patent Application DE 198 25 501 A1).
Furthermore, a device can be taken from International Patent Application Publication WO 2013/167384 A1 and corresponding U.S. Patent Application Publication Patent 2015/082881 for detecting a limit level of a medium, in which the frequency of an output signal of a resonator is evaluated. The resonator is in contact with the interior of the chamber that contains or conducts the medium. Detection thereby occurs in that the frequency of the resonator is exploited depending on the dielectric constant of the medium. In one design, the resonator is designed as a micro strip line or as an emitting device.
It is known from a completely different field of application to determine the moisture of objects in a container using reflector switches, see German Patent Application DE 10 2004 016 725 A1.
A strip line antenna for fill level recognition is, for example, provided in German Patent Application DE 199 35 743 A1. Thereby, in one design, two receivers for microwave signals are used for the redundant monitoring of the fill level.
An exemplary design of emitting devices that are used in the field of fill level measurement is disclosed in German Patent Application DE 10 2006 019 688 B4 and corresponding U.S. Pat. No. 7,710,328. Such emitting devices consist in general of metal surfaces that are present on or in printed circuit boards or other substrates.
It is additionally known in the prior art to further add metal surfaces acting as reflectors.
So-called patch antennae are also known in the prior art (other terms are “flat antenna” or “micro strip antenna”). An array is created by interconnecting several antennae. It is thereby known to create different resonance structures: half-wave or quarter-wave, see e.g., German Patent Application DE 699 36 903 T2 and corresponding U.S. Pat. No. 6,218,990.
Panel antennae can be used for measuring the surface structure of media, see e.g., European Patent Application EP 1 701 142 A2 and corresponding U.S. Pat. No. 7,408,501.
Panel antennae that are arranged in ceramic structures are used for fill level recognition, for example according to International Patent Application Publication WO 2009/121530 A1 and corresponding U.S. Pat. No. 8,474,314.
In the fill level switch according to European Patent Application EP 1 956 349 A2 and corresponding U.S. Pat. No. 7,730,780, electromagnetic signals are coupled into each of a measuring conductor arrangement and a resonance conductor arrangement, wherein the signal of the measuring conductor arrangement can interact with the medium, whose fill level is to be monitored.
A device having microwave sensors for the specific use of monitoring the fill level of a blood reservoir is described in German Patent Application DE 195 16 789 B4. These sensors are attached outside of the reservoir and radiate through its wall. It is described, in one design, that several sensors are attached at the same level. It is further provided that sensors are attached at different levels in order to already be able to signal that the fill level is approaching a critical limit level.
Contaminants or the deposition of medium on the container wall or especially on the sensors are problematic for the above-mentioned devices or, respectively, limit level switches. This can lead to false signals or can even prevent the detection of the reaching or falling below a fill level.
Originating from these problems, it is known, for example, in membrane oscillators to shake off gas bubbles during a cleansing phase (see German Patent Application DE 10 2008 050 445 A1).
In safety-critical uses, it is further known to design the sensors or devices redundantly. In the simplest case, two measuring devices are used that have the same function.
In particular, the following types of redundancy are thereby differentiated:
In hot spare, several sensors are actively operated. A so-called voter evaluates the individual results, wherein, if necessary, the majority decides.
In cold redundancy, several sensors are present, however, only one sensor is active. Based on the evaluation of the signals of this individual sensor, further sensors are activated, if need be.
In standby or passive redundancy, again, several sensors are present and, again, only one sensor is active. The non-active sensors are thereby in a standby state. In the case of an error of the active sensor, another sensor is switched on.
Thereby, the redundancy can be homogeneous or diverse. This means, for example, that either the same type of sensor or the same components are multiply used, or that the individual sensors or components come from different producers or are based on different principles.